I would like some feedback on the idea to use a node type a bit heavierthen SPV (dubbed FSPV) to solve Fraud Proofs.

An FSPV node not only downloads block headers, but also the "spend-treeblocks", consisting of all TXIDs and all previous output indices andTXIDs. The latter can be compacted using a scheme similar to CompactBlocks, which will make the spend-tree block ~80kb in size.

ThIs way the FSPV can track the full transaction graph at little cost.

The advantage is, that Fraud Hint messages for absent/withheldtransactions become feasible. A normal SPV is reduced to Full Node bysuch (cheaply faked) hint, but for an FSPV the cost is almost zero.

All it needs to do is add a taint-bit in the tree and propagate thetaint to the transaction graph. It then knows it needs to request theFraud Hinted transaction to consider any descendant transaction valid.

This makes it sufficient to punish fraudulent fraud hints or withheldtransactions by normal "banscore" procedures.